The ZWay plugin uses the high-level interface to the ZWay firmware. As such, it only has access to those things which that API provides. It seems somewhat random sometimes. Through this interface we are seeing the sensor in question, but it's an issue to do with how we represent these as child devices. However, the 'raw' variable in the parent device
zway_71-0-49-4 is available for you to read, but you can't set a variable watch on it. You'd have to poll it with a timed scene, or such like.
I will look further into adding this to the child device, but, as mentioned, there may be a conflict with other reported power variables. We'll see.
Yes, there is potential conflict. Just checked the source code and find this comment:
-- 2020.03.05 "Watts" conflicts with meter "50-2" if both present
I may, in general, be able to code around this in the event that there is no reported command class 50-2, but for this device there IS, so that's a problem.